This application relates to and claims priority to corresponding German Patent Application No. 100 50 125.7 filed on Oct. 11, 2000.
The invention relates to an apparatus for compensating temperature differences in thermally loaded bodies of low thermal conductivity. The invention relates also to apparatuses for mirror supports which are used in projection lenses for micro lithography.
Heat-distributing apparatuses are generally known in micro lithography. Thus, for example, U.S. Pat. No. 5,220,171 and U.S. Pat. No. 5,413,167 describe heat-distributing apparatus for cooling wafers by using a liquid guided in channels as heat-transporting medium.
U.S. Pat. No. 5,313,333 relates to devices for compensating temperature deviations in an optical assembly.
Japanese publication JP 83 13 818 discloses a mirror with a cover plate, which has a reflecting layer, and a lower plate which is arranged therebelow and is situated at a spacing therefrom, the two parts being connected to one another by webs and chambers situated therebetween.
A mirror support in a projection lens for micro lithography is heated by absorbed useful radiant energy. This results in two temperature problems which are to be distinguished from one another:
One problem consists in an excessively large increase of the mean substrate temperature. This can lead to variations in the micro-structure of substrate and layer materials, which disadvantageously affect the properties of the optical surface.
The other problem resides in an inhomogeneous temperature distribution inside the mirror substrate. Because of the thermal expansion, an inhomogeneous temperature distribution results in an expansion distribution which is approximately analogous thereto and deforms the mirror substrate and thus the optical surface.
A solution should be found for both thermal problems, but for that of the inhomogeneous temperature distribution, in particular. The problem of the inhomogeneous temperature distribution will be explained in more detail below to promote better understanding of the particular mode of operation:
The inhomogeneous temperature distribution inside the substrate essentially has the two following causes:
Whereas the heat input takes place virtually exclusively via the optical surface, the output of heat is accomplished chiefly by emission at the edge and rear of the mirror, and partially by thermal conduction via the mount. Since the points of heat input are therefore situated elsewhere than those of heat output, temperature gradients are formed because of the thermal resistance of the substrate material.
The second cause is the illumination, generally inhomogeneous, of the optical surface, because firstly the radiation region does not occupy the entire optical surface, and secondly the circuit pattern projected with the aid of the projection optics causes an inhomogeneous intensity distribution inside the radiation region. Strongly irradiated regions of the optical surface then warm up more strongly than weakly irradiated ones. If it is desired to solve the problem of the substrate deformation caused by an inhomogenous temperature distribution, it is obvious to use a substrate material whose coefficient of thermal expansion is very low. This approach is often adopted in precision optics by selecting substrate materials such as quartz, Zerodur or ULE, and leads to a substrate deformation which is sufficiently low for many applications. However, a disadvantage of the said materials is the thermal conductivity, which is much lower by comparison with metallic materials and leads to comparatively large temperature differences inside the thermally loaded mirror support and partially cancels out again the deformation-reducing effect of the low coefficient of thermal expansion. This fact has a very disadvantageous effect particularly in the case of mirror supports in micro lithography lenses for the 13 nm technology (EUVL), since because of the high degree of absorption of an individual optical surface of approximately 40% in the 13 nm band, the heat flux in the mirror support becomes very large and large temperature differences thereby occur in the substrate. At the same time, the requirements placed on the accuracy of the surface shape in a mirror system such as is represented by an EUVL lens are substantially more stringent than in the case of lens optics such as are chiefly used at present in micro lithography.
It is therefore the object of the present invention to create an apparatus by means of which the heat distribution in the thermally loaded body can be improved without the risk of thermal deformations and without simultaneously worsening the low coefficient of thermal expansion.
This object is achieved according to a heat-distributing device having one or more heat-distributing bodies adapted to surfaces of the thermally loaded body such that there remains between the thermally loaded body and the heat-distributing bodies a gap which is filled with a fluid for the purpose of the thermal coupling of thermally loaded bodies and heat-distributing bodies in conjunction with mechanical decoupling.
According to the invention, a separation now takes place between a mechanical coupling and a thermal coupling with reference to the thermally loaded body. A coupling fluid is introduced in an appropriately created gap or in an intermediate gap between the heat-distributing device, which distributes heat and dissipates heat and is, for example, embedded in the thermally loaded body or arranged thereon, and the thermally loaded body. The coupling fluid ensures thermal coupling to the heat-distributing device, but simultaneously decouples the latter mechanically from the thermally loaded body. In this way, deformations of the heat-distributing device are not transmitted onto the thermally loaded body, for example a mirror substrate or a mirror support.
In this case, a solid body made from a material of high specific thermal conductivity such as, for example, Cu, Al, Ag etc. can be used as heat-distributing body. Another embodiment of a heat-distributing body consists of a thin-walled solid body penetrated by capillaries, for example a tube, through whose capillaries a second fluid flows. Here, the heat distribution takes place through the entrainment of the heat with the flowing fluid.
The coupling fluid, which fills the gap between the substrate, that is to say the thermally loaded body, and the heat-distributing body, can be a liquid, a gas or else a material of sufficiently low viscosity. Preference is given to liquids of good thermal conductivity such as, for example, water, mercury or metal alloys which are liquid at room temperature.
In order to rule out a deforming influence of the pressure of the coupling fluid on the thermally loaded body, the heat-distributing device can include a device for pressure compensation between the coupling fluid and the external surroundings of the thermally loaded body. The device can be designed in the form of an ascending pipe or an elastic vessel, for example a metal diaphragm bellows. Preference is given to the design having an elastic vessel formed from a metal diaphragm bellows, since this renders it possible to seal the coupling fluid with reference to the surroundings of the substrate, and thus to prevent the coupling fluid from running out or outgassing.
The coupling fluid, which fills the gap between the thermally loaded body and the heat-distributing body, executes no movement, that is to say does not flow. This rules out pressure differences inside the gap volume on the basis of flow pressure drops.
The apparatus according to the invention ensures a substantial reduction in the temperature difference inside the thermally loaded body, and thus ensures a reduction in the thermally induced deformations of the optical surface, the high degree of mechanical decoupling in the heat-distributing device and the thermally loaded body taking account, in particular, of the high requirements placed by micro lithography on the dimensional stability of the optical surfaces.
In a preferred development of the invention, the heat-distributing device is connected to one or more temperature controllers. It is possible with this development to reduce and stabilize the mean temperature of the thermally loaded body. It is possible, for example, to use as temperature controllers Peltier elements whose cooling side is directly connected to the heat-distributing body of the heat-distributing device, and whose warmer side outputs the heat absorbed by the thermally loaded body, and the lost energy occurring during operation of the Peltier element, doing so by thermal radiation. In another embodiment, the heat-distributing body is flowed through by a cooling liquid which is guided out of the heat-distributing body and is cooled by a temperature controller situated outside the heat-distributing device.